Sponge iron or Direct-reduced iron (DRI) is formed when naturally available iron ore which is an oxidized form of iron (magnetite (Fe3O4) or hematite (Fe2O3)) is reduced to a metallic form by a reduction process. The reduction process occurs below a melting temperature of both metallic iron and the oxidized form. During the reduction process a large amount of material gets eliminated during oxygen removal. Oxygen removal from iron ore creates multiple microscopic pores in the iron ore. The microscopic pores give the iron ore a sponge like texture and hence it is termed as sponge iron. Due to high purity, the sponge iron is an excellent feedstock for electric furnaces used by various mini mills.
Typically, in industrial applications, a rotary kiln is utilized for formation of the sponge iron. The rotary kiln is a cylindrical vessel, inclined slightly to the horizontal which is rotated slowly about an axis. Mixture of iron ore, coal and Dolo is fed into the upper end of the cylinder. As the rotary kiln rotates, the materials gradually moves down towards the lower end, and may undergo a certain amount of stirring and mixing. Hot gases pass along the rotary kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current).
Over a period of time some of the input materials stick with the wall of the rotary kiln called accretion. The accretion starts to block the flow of materials within the rotary kiln. Gradually thickness of the accretion increases and at some point of time the accretion totally blocks the path of flow of materials inside the kiln. Then the operation has to be shut down and the rotary kiln has to be cleaned. Each shout down reduces production. If location of accretion inside the rotary kiln can be spotted at an early stage then the accretion can be broken by certain methods in the running kiln and shut down can be avoided.